1. Field of the Invention
The invention relates to a cryogenic fluid pump that is capable of continuously recirculating a cryogenic fluid for several weeks without user intervention.
2. Background of the Invention
Liquid nitrogen and other cryogenic liquids are used in a variety of scientific applications to cool experimental systems. For instance, cryogenic liquids provide favorable kinetics, confer improved vacuum conditions, and reduce the amount of contaminants in experimental procedures.
Some experiments require a constant flow of a liquid cryogenic at a low rate. A “low rate” is generally considered to be a rate less than 10 L/min. Many currently available cryogenic pumps are unsuitable for continuous operation at this rate. Further, not all pumps can be adapted to operate at a flow rate lower than their designed flow rate. Centrifugal pumps in particular are not designed to operate at rates much higher or lower than their manufacturer's stated best efficiency flow rate. Therefore, providing a steady stream of cryogenic liquid to some experiments is a challenge.
One example in which this cryogenic pumping challenge is particularly acute is ion-trapping experiments, such as Paul Traps. In Paul Traps, buffer gases are used to aid in ion transport and confinement. Bringing the buffer gas from room temperature to the temperature of a cryogenic liquid significantly reduces the spatial spread of trapped ions and improves the gas purity in the cooling region. These improvements increase the trap storage times. Longer trap storage times would allow for more accurate observation of the trapped ion. In some instances, meaningful observation can take days or weeks, but providing the small, constant flow of cryogenic liquid required for these long-term experiments has proven difficult.
Typically, the cryogenic liquid is continuously pumped by self-pressurization into the experimental setup and then lost downstream. Self-pressurization pumps use the increasing gas pressure caused by the boiling liquid inside the sealed Dewar container to push the liquid downstream. The pressure gradient driving the flow is lost in conventional methods during attempts to re-collect the liquid. Recirculation requires liquid to flow both into and out from the reservoir, but self-pressurization pumps can only support outward flow. Since the cryogenic liquid cannot be recovered, a very large supply of cryogenic liquid is required for the duration of the experiment. Obtaining and maintaining such a large supply of cryogenic liquid can be difficult, especially in small laboratory setups. Additionally, it also wastes material and effort.
Supplying the cryogenic liquid to the experimental device also provides its own difficulty. Because cryogenic liquids are boiling, they cannot be pulled through a circuit by a downstream pump. Instead, they must be driven by a positive-pressure device that is submerged in the liquid. The types of positive-pressure devices that can withstand cryogenic temperatures are limited to centrifugal pumps inasmuch as such pumps do not contain flexible components that become brittle when exposed to extreme cold. However, centrifugal pumps are susceptible to cavitation, which is further exacerbated by the fact that cryogenic liquids are constantly boiling.
Cavitation occurs when vapor cavities, i.e., gas bubbles, form in a liquid as a result of forces acting on the liquid and subsequently collapse against the impeller vanes. One prominent reason for cavitation is the rapid change of pressure in the liquid, such as when a liquid experiences a steep pressure drop when it reaches the eye of the impeller in a pump. The pressure drop is caused by a decrease in flow area, which causes an increase in flow velocity. Cavitation in a pump causes large amounts of noise, vibration, pressure pulsation, degradation of pump components, and loss of efficiency. If the pump chamber is sufficiently filled with gas bubbles, the flow of liquid will cease entirely.
The problems of pump cavitation are further exacerbated if a continuous flow at a low rate is desired or if the flow resistance is high. Evaporation of the cryogenic liquid in the flow circuit creates high flow resistance. Therefore, a need exists in the art for a cryogenic liquid pump that is capable of providing continuous flow for a period of days, weeks, or months at a low flow rate without the attendant cavitation problems that beset other pumps.
State of the art liquid nitrogen pumps are commercially available. However, these pumps are large and extremely expensive. Further, they are unsuitable for cooling small laboratory equipment.
Thus, a further need exists in the art for a compact cryogenic pump that is relatively inexpensive and is suitable for small-scale laboratory experimentation. The pump should also supply cryogenic fluids at rates less than 10 L/min.